This invention relates to a process for recovering methanol solvent used in the production of 4,4'-isopropylidenebis(2,6-dibromophenol) better known as tetrabromobisphenol-A, hereinafter referred to as TBBPA.
TBBPA is a well known commercial flame retardant. Products comprised predominantly of TBBPA are useful as flame retardants in many macromolecular formulations. The preparation of TBBPA by brominating bisphenol-A in a solvent is well-known. The bromination agent typically is bromine or a bromine-chloride mixture. The solvent can be an alcohol, aqueous acetic acid, a non-polar solvent or a two-phase water-organic system, but is preferably an aliphatic alcohol, most preferably a lower aliphatic alcohol such as methanol or ethanol. The literature is replete with processes for the manufacture of TBBPA, see, for example, U.S. Pat. Nos. 3,029,291; 3,182,088; 3,234,289; 3,363,007; 3,546,302; 3,868,423; 3,929,907; 4,013,728; 4,036,894; 4,112,242; 4,180,684; 4,431,847; 4,451,675; 4,628,124; 4,701,568; 4,783,556; 4,909,997; 4,990,321; 5,008,469; 5,017,728; 5,059,722; 5,059,726; Japanese Kokai 2 (1990) 196,747; EPO 380,363; and British Patent 949,306 incorporated herein by reference as if fully set forth. Most if not all of the foregoing processes describe the recovery of TBBPA from the reaction medium by adding water to precipitate the product as product crystals. The crystals are filtered, washed on the filter to remove impurities and dried. The filtrate from the filter is collected and treated to recover solvent for reuse. Since the filtrate contains HBr and/or bromides, it is desirable to recover any bromide containing species from the filtrate for their bromine value. Bromides may be formed as a result of neutralization of the filtrate stream which contains HBr resulting from the ar-bromination of bisphenol-A.
If a methanol solvent is used, as exemplified in U.S. Pat. No. 3,128,088, the excess solvent may be recovered from the filtrate stream upon completion of the reaction using a methanol distillation column. Depending on the operating conditions selected for the distillation process, a variety of compounds may be present in the column bottoms. Typically, methanol distillation is carried out under basic conditions using an alkali metal hydroxide such as NaOH at a pH of 10-14. Under basic conditions the components present in the distillation column bottoms include sodium salts of brominated phenolics, residual methanol, sodium bromide, trace amounts of acetone, NaOH, water and the like. While the average composition of the distillation column bottoms is quite variable, a typical composition for a distillation process having a pH above 9.5 is 83.75-95.4 weight percent water, less than 0.2 weight percent methanol, 0.9-5.0 weight percent brominated phenolic compounds, 3.0-15.0 weight percent NaBr and 0.1-1.0 weight percent NaOH.
In U.S. Pat. No. 4,431,847, a process is described which removes polyhalogenated phenolic compounds from the distillation column bottoms streams such as those described above by forming an insoluble solid polymer that is easily removed by filtration or centrifugation. This solid polymer is then disposed of by landfill. Due to the increasing undesirability of the use of landfills for industrial waste streams, there is a need for a more environmentally acceptable and less costly means for recovering phenolic compounds from the filtrate and distillation column bottoms streams produced in the production of tetrabromobisphenol-A.